Instead of using a parent structure that has to be derived by API
consumers, this change propse a simplification by using the common
pattern of GTK of passing a function pointer and user data which will
be passed as its parameter. That user data contains the state and the
function will be called to update that state.
This new API allows the user to call a single method (process)
which handles the [display] lock/unlock logic internally for
them.
This API supersedes the risky begin, render, end API.
It eliminates the need for the user to call a lock method
(process_begin) before processing the input buffers
(process_render) and calling an unlock method (process_end)
afterwards.
See #219
